Human eye movement includes rapid and minute movement (small involuntary eye movement), and the involuntary eye movement is categorized into the following three types of movement. That is, the three types of movement include drift that is small smooth movement at low frequency, flick that is small saccadic movement (also called microsaccade), and tremor that is very small high frequency oscillation. It has been said that the magnitude of involuntary eye movement varies according to tiredness, illness, and the like, and it has been reported, for example, that the magnitude of drift of a patient with dementia is different from that of a healthy person, and moreover, because tremor is not measured immediately before brain death, it has been considered that there is a close relationship between tremor and brain function.
There have been many attempts at measuring such small involuntary eye movement and applying the involuntary eye movement to estimate a human's state of consciousness and state of health. It has been described, for example, in Patent Document 1 that the degree of visual attention when gazing at a high-definition screen or the like can be found based on the magnitude of the involuntary eye movement. In addition, an eye control system information detection apparatus described in this document, for removing a tremor component that is a high-frequency component and becomes noise when extracting from the involuntary eye movement a drift component and a microsaccade component that are low-frequency movements, smoothens time series data concerning an eye movement.
Moreover, while a tremor component has been removed as noise in Patent Document 1 described above, information that can be estimated based on the tremor component has also been suggested. For example, in Non-Patent Document 1, the relationship between the tremor component and the brain stem has been suggested, and there has been proposed a contact measurement apparatus using a piezoelectric element, for applying a tremor component to an index to find the depth of sleep, coma, or anesthesia, an index of brain death, and the like. Moreover, in Non-Patent Document 2, it has been reported that a tremor component of a patient with Parkinson's disease is unique.    Patent Document 1: Japanese Patent No. 3724524    Non-Patent Document 1: N. F. Sheahan et al., “Ocular microtremor measurement system: design and performance”, Medical and Biological Engineering and Computing, 31, pp. 205-212 (1993)    Non-Patent Document 2: Ciaran Bolger et al., “Ocular microtremor in patients with idiopathic Parkinson's disease”, J. Neural Neurosurg Psychiatry, 66, pp. 528-531 (1999)